Field of the Invention
The present invention relates to a capacitive transducer used as an ultrasonic electromechanical transducer or the like, and also relates to a method for manufacturing the capacitive transducer.
Description of the Related Art
Micromechanical members manufactured by micromachining can be processed in the order of micrometers. Various micro-functional devices are implemented by such micromechanical members. Capacitive transducers using such technology have been studied as substitutes for piezoelectric devices. Such a capacitive transducer is capable of transmitting and receiving acoustic waves, such as ultrasonic waves, by using vibration of a vibrating film (acoustic waves may be hereinafter referred to as ultrasonic waves). The capacitive transducer can easily achieve good wideband characteristics particularly in liquid. As used herein, the term “acoustic waves” includes those referred to as sonic waves, ultrasonic waves, and photoacoustic waves. For example, the term “acoustic waves” includes photoacoustic waves generated in a subject by irradiating the inside of the subject with visible light or infrared light (electromagnetic waves).
Japanese Patent Laid-Open No. 2008-98697 proposes a capacitive transducer related to the technology described above. This capacitive transducer is made by removing a sacrificial layer by means of wet etching through etching holes, and performing filling-up sealing which involves sealing the etching holes with an insulating film. PCT Japanese Translation Patent Publication No. 2007-528153 discloses another capacitive transducer. This capacitive transducer is made by removing a sacrificial layer by means of wet etching through etching holes, performing filling-up sealing which involves sealing the etching holes with an insulating film, and then etching the insulating film, so that the thickness of a vibrating film can be adjusted to achieve desired resonance frequencies.
In the configuration disclosed in Japanese Patent Laid-Open No. 2008-98697, the entire thickness of a layer for filling-up sealing of the etching holes used to remove the sacrificial layer is added to the thickness of the vibrating film. This increases the thickness of the vibrating film and narrows the frequency band accordingly.
With the technique disclosed in PCT Japanese Translation Patent Publication No. 2007-528153, a capacitive transducer having a wide frequency band can be made by forming a thin vibrating film. However, reducing the thickness of the vibrating film by means of etching may cause variation in the thickness of the vibrating film due to variation in etching rate in the substrate. This leads to variation in frequency characteristics and transmission and reception sensitivity of the capacitive transducer.
A thin vibrating film tends to deform significantly under stress or depending on the thickness. Since this makes it difficult to create a narrow gap, it is not easy to improve the sensitivity of the capacitive transducer.